Television system



Feb. 18, 1941'.

A. v. BEDFQBD TELEVISION SYSTEM Filed Nov. 28, 1936 4 Sheets-Sheet 1 :inventor Feb. 1s, 1941.

A. V. BEDFORD y TELEVISION SYSTEM LIPPING LE VEL v Y:inventor Hld V.' Bedfo rd El* y Gtforneg Feb. 18, 1941- A. v. BEDFORD -TELEVISION SYSTEM Filed Nov. 28, 1936 4 Sheets-Sheet 3 QW WW Snnentor lf. Be dfo ral (Ittorneg H da Bg y n @El Feb. 18, 1941. l A v BEDFORD 2,232,044

TELEVIS ION SYSTEM Filed Nov. 28, 1936 4 Sheets-Sheet 4 Snnentor HzdayBed-ford v Bg y A, (Ittomeg Patented Feb. 18, 1941 1 UNITED STATES TELEVISION SYSTEM Alda V. Bedford, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of .Delaware Application November l28, 1936, Serial No; 113,130

1 Claim.

My invention relates to Atelevision systems and particularly to systems of the type employing cathode ray transmitter tubes and cathode ray receiver tubes.

In television systems of this type, the cathode ray in the transmitter tube scans a photoelectric screen to produce picture signals While the cathode ray in the receiver tube scans a fluorescent screen to produce a picture thereon corresponding to said signals. Scanning at both transmitter and receiver is accomplished by deflecting the cathode ray horizontally by means of a sawtooth Wave recurring at a comparatively high frequency and by simultaneously deiiecting, it vertically by means of a saw-tooth wave recurring at a comparatively low frequency. Synchronization of scanning at the transmitter and at the receiver is maintained by transmitting a synchronizing impulse at the end of each scanning line and at the end of each picture frame.

It is the usual practice to prevent the production of a return line trace on the fluorescent screen of the cathode ray receiver tube by ycutting-off or blocking the cathode ray during the return line period, both for horizontal and vertical deflection. It has .also been found advisable to cut off the cathode ray in the transmitter tube during the return line period for both horizontal and vertical deflection. This prevents the cathode ray from partially discharging the photoelectric screen during the return line period. I have found that the above mentioned blocking of the transmitter tube cathode ray creates an electrical disturbance Which itis diflicult to eliminate entirely at the transmitter.

It is, accordingly, one object of my invention to provide a method of and means for eliminating at a television receiver substantially all disturbing eifects due to blocking of the cathode ray in the transmitter tube.`

A further object of my invention is to provide a method of and means for insuring that the transmitter tube cathode ray is blocked for the entire duration of a return line period.

More broadly, an object of my invention is to provide a method of and means for avoiding undesirable eliects at a cathode ray receiver tube due to the return deflection of the cathode ray in the transmitter tube' In practicing my invention I apply a rectangular voltage impulse to the control grid of the cathode ray Vtransmitter tube to block the ray during the return line period and I make this impulse of greater duration than the driving or synchronizing impulse which is supplied to the transmitter tube deflecting circuit. Also, I transmit to the receiver a rectangular voltage impulse which blocks the ray `of the cathode ray receiver tube during the return line period, and I make (Cl. FX8-6.8)

this impulse of greater duration than the 'cathode ray blocking impulse at the transmitter.

My invention Will be better understood from the following description taken in connection With the accompanying drawings in Which- Figs. 1 and 2 are blockldiagrams of a television transmitter and of a television receiver, respectively, in a system embodying my invention,

Fig. 3 is a group of curves which are referred to in explaining my invention,

Figs. 4a to 4c are curves which are referred to in explaining my invention,

Fig. 5 is a circuit diagram of a portion of the transmitter shown in Fig. l, and

Fig. 6 is a group of curves that are referred to in explaining the operation of the circuit shown in Fig. 5.

Referring to Fig. l, there is illustrated a television transmitter of the general type described and claimed in my copending application, Serial No. 728,147, filed May 29, 1934, and assigned to the Radio Corporation of America. The impulse generator indicated at I may be of the type employing rotating discs as described in the said copending application, but preferably it is of the type described and claimed in Patent No. 2,132,655, issued October 11, 1938, in the nam'e of John P. Smith and assigned to the Radio Corporation of America.

As described in the said Smith application, the!` generator I may be designed to supply synchronizing or driving impulses through conductors 2 and 3 to the horizontal and vertical deiiecting circuits 4 and 6, respectively, of the cathode ray transmitter tube 1. Also, as shown in the Smith application, the generator I may supply blanking impulses through a conductor 8 and receiver s chronizing impulses through a conductor 9.

The transmitter tube I is of a Well known type comprising an evacuated envelope having therein an indirectly heated cathode II, a control grid I2, a first anode I3, a second anode I4 and a photoelectric screen or mosaic I6. The screen I6 may consist of a sheet of mica or other insulating material I1 having a metallic surface I8 on the back side and having a mosaic of microscopic globules I9 on the front side. The globules are light-sensitive elements such as silver globules coated with -caesium and are insulated from each other as Well as from the surfaceor back plate I8.

The electrodes of the tube 1 are maintained at suitable potentials by means of connections to the voltage divider 2I of a power supply unit (not shown), the control grid I2 being connected to a negative point on the voltage divider through a resistor 22.

The metal surface or back plate I3 is connected through an output resistor 23 to ground. When the image of a scene to be transmitted is be described more fully, and as is describedin` my above mentioned application, the picture sig`y nals are properly combined with blanking and synchronizing impulses beforeY they 'are *trans-A mitted, the blanking signals being supplied from the conductor 'through an amplifier 21 and a conductor 28 to one stage of the amplifierv 24, and the synchronizing signals being V`supplied from the conductor 9 through the amplifier- 29 and the conductor 3| to a succeeding stage'in amplifier 24. y

In order to block the cathode ray of the transmitter tube 1 during the return line period, the impulses supplied over conductors 2 and'3 are added through resistors 32, 33 and 34 andimpressed upon the control grid I2 through an amplifier 36.

In accordance With one featureof my invention, the blanking impulsesV which are impressed upon control grid I2 are wider, that is, of greater duration, than the driving impulses supplied to the transmitter tube defle'cting circuits 4 and 6. For convenience, and in' order to make the block I correspond exactly to the disclosure of theabove mentioned Smith application, I have shown a circuit wherein the impulses supplied over conductors 2 and 3 are employed without any change in width for blanking and are narrowed by shaping networks'31 and 38,'respectively, before being impressed upon the deiiecting circuits.

The shaping networks 31 and 38 may be designed in several'ways, one satisfactory design being shown in Fig. 5, which is a circuit diagram of the network 31. Referring to Fig. 5,r this particular network includes four resistance coupled vacuum tubes 39, 4|, 42 and 43. The rectangular impulses indicated at 4|) which are supplied over conductor 2 (Fig. 1) are impressed upon the input circuit of the vacuum tube 39 through a coupling condenser 44 and a resistor 45 of comparatively high resistance.` The input circuit of the tube 39 includes a resistor 41 of comparatively high resistance connected between the grid and the cathode of the tube 39, this'resistor being shunted by a condenser 48 of comparatively large capacity. The circuit including the resistance 41 and condenser48, which may be referred to as an integrating circuit, causes the rectangular impulse 40 to acquire the shape indicated at 49. The reshaped impulse 49 is then clipped by the tube at the level indicated by the dotted line 5| .to produce in the output circuit `oi' the tube 39 an impulse'of the shape indicated at 52. It will be understood that the grid of the tube 39,and the grids of the other tubes in the circuit which function to clip the impulses, is maintained at the desired negative bias by means of the well' known grid leak biasing action.

The tube 4| is employed merely for the purpose of reversing the polarity of the impulse to produce the impulse indicated at 53. This impulse is then impressed upon what may be described as a diiferentiating circuit comprising a coupling condenser 54 of very small capacity and a grid resistor 56 of comparatively low resistance. The resulting impulse appearing across the input electrodes of the tube 42 has a wave shape indicated at 51. Thereason that the impulse has been reshaped in this wayis that 'it' has been j narrowed impulse indicated at 62.

The action of the circuit shown in Fig. 5 may be better understood by referring to Fig. 6, where impulses corresponding to those shown in Fig. 5 are indicated by the same reference numerals. It will-be seenthat the incoming rectangular impulse. 40 is caused by the integrating circuit to have a sloping front side of increasing amplitude for the duration of the original impulse and a slopingfback side of decreasing amplitude beginning at the termination of the original rectangular impulse. By clipping this resulting impulse 5I at a certain level, an impulse of the desired width may finally be produced. After the impulse 5| is clipped, it has the shape indicated at 53. By passing impulse 53 through a diiferentiating circuit, the gradually sloping f front side of the impulse causes a narrow substantially rectangular portion to be produced in the resulting impulse 51. It will be noted that the flat clipped portion of the impulse 53 causes a corresponding at .portion to appear in the impulse 51, since the amplitude of the signal is not changing in this region. It is apparent that, by clippingthe impulse 51 as indicated, the desired narrow impulse |52 is obtained. Figs. 4a to 4e show the relative widths of the horizontal driving and blanking impulses. The impulses 62 shown in Fig. 4a are the impulses which appear in the output circuit of the impulse `shaper 31, these impulses occurring in the specific transmitter being `described at the rate of 10,290 per second. Each of these impulses is the same as the impulse 6 2 shown in Fig. 5, the impulse in Fig. 5 being shown with vertical sides for the purpose of simplifying the drawing. It will be understood that it Ais practically impossible to obtain a rectangular impulse having sides which do not slope at all, or to transmit it through the circuits if it is obtained. Fig. 4b shows the saw-tooth current wave which passes through the lhorizontal deflecting coils of the transmitter tube 1 as a result of the horizontal driving impulses being'impressed upon the deiiecting circuit 4. It will be noted that, after the termination of the driving impulse 62, considerable time'is required for the cathode ray of the tube 1 to complete its return and start upon f the linear portion, that is the useful portion, of its scanning cycle. This is due to the inductance of the delecting yoke and to the time required to charge distributed capacities of the circuit and also to the delay in transmission through the cable which connects thev deflecting circuits to the delecting coils. Hence, the duration of the transmitter tube driving or synchronizing impulses 62 should be shorter than the transmitter tube blanking impulses in order that the blanking may include all of the' return period of the transmitter tube.

Ihe transmitter tube blanking impulses, which may be 4the impulses 40 unchanged in width, are shown in Fig. 4c. It will be seen that they have a width' or duration enough greater than that of the driving impulses to block the cathode ray completely during the return line period. .-Actually, the transmitter tube blanking impulse, as it appears at the control grid I2 of the transmitter tube, Will have the shape shown in Fig. 4d, this shape being due largely to distributed capacity in the circuit.

The horizontal blanking impulses 66 which are transmitted to the television reciver for blanking out the receiver cathode ray during the return line period are shown in Fig. 4e. These impulses have a greater width or duration than the blanking impulsesl shown in Fig. 4d, whereby any disturbances produced at thetransmitter as the result of blocking the cathode ray are prevented from appearing on the fluorescent screen of the receiver tube.

It may be noted that if the blanking impulses at the transmitter and receiver are of the same Width, the disturbance produced by blocking the transmitter ray may not be included (in time) by the receiver blanking impulse due to the slight delay in the transmission of the transmitter tube blanking impulse through a cable to the cathode ray tube and to the inadequate steepness of the Wave fronts of the impulses actually reaching the respective control grids of the cathode ray transmitter and receiver tubes.

The above mentioned considerations apply to the widths of the vertical synchronizing and blanking impulses, as well as to the Widths of the horizontal impulses. This will be seen by referring to Fig. 3, wherein the nature of the synchronizing and blanking impulses is more completely illustrated. One of the vertical synchronizing or driving impulses for the transmitter tube 1 is indicated at 63. This impulse is supplied from the conductor 3 (Fig. 1) through the network 38 to the vertical deflecting circuit 6. The vertical synchronizing impulses occur at the rate of 60 per second in the specific transmitter being described. 'I'he horizontal synchronizing or driving impulses for the transmitter tube indicated at 62 are the same as those shown in Fig. 4c, the impulses being shown with straight sides in order to simplify the drawings.

One of the vertical blanking impulses which are supplied through the amplifier 36 to the grid I2 is indicated at 64. These impulses are wider than the vertical driving impulses 63, as will be seen from an inspection of the curves. In this figure the difference in width of the horizontal synchronizing impulses 62 and the horizontal blanking impulses 40 is not shown.

The combined horizontal and vertical blanking impulses which are transmitted over the conductor 8 to the picture amplifier 24 for transmission to the receiver are shown under the legend "Receiver blanking. It will be seen that the vertical blanking impulse 66 for the cathode ray receiver tube is of substantially greater duration than the transmitter tube blanking impulse 54. The complete signal which modulates the carrier of the radio transmitter 26 is indicated at the bottom of the iigure, the horizontal and vertical blanking impulses being indicated at 60 and 65, respectively, and the horizontal synchronizing and the slotted vertical synchro- Anizing impulses which are set on. top of the blanking impulses being indicated at 61 and 68, respectively. The picture signal appears between the blanking impulses 60, as indicated at 69, black in the picture being represented by the top of the blanking impulse or pedestal, indicated at 1|.

While I have shown the vertical driving or.

synchronizing impulses and the vertical blanking impulses as beginning at the same time, it should be understood that it may be desirable to have the transmitter tube blanking signals begin slightly ahead of the transmitter tube driving impulses and it may also be desirable to have the receiver tube blanking impulses begin slightly ahead of the transmitter tube blanking impulses as is indicated in Figs. 4a to 4e for the horizontal driving and lblanking impulses.

The several impulses may be made to start one ahead of the other by employing any suitable delay network, transmission line, or the like for delaying certain impulses. For example, delay networks 30 and 4D may be included in the horizontal and vertical deiiecting channels for delaying the driving impulses a certain amount. A delay circuit or network may be included in the amplifier 36 for delaying the blanking impulses or the tube 1 a less amount while the blanking impulses for the receiver may be transmitted with substantially no delay.

The receiver shown in Fig. 2 may be of a Wellknown type including a cathode ray tube 12 having horizontal and vertical deilecting coils. The picture, blanking and synchronizing signals, shown by the bottom curve in Fig. 3, are impressed upon the control grid 13 of the cathode ray tube through a radio receiver 14 and picture amplifier 16. This same signal is supplied to a separating circuit 11 where substantially all of the picture signal is removed and the horizontal and vertical synchronizing impulses separated and supplied to their respective deecting circuits 18 and 19. It will be understood that these deecting circuits are of any suitable type for producing saw-tooth deflection of the cathode ray.

The cathode ray tube itself may be of conventional design including an evacuated member having therein an indirectly heated cathode 8|, the control grid 13, a iirst anode 82, a second anode 83, and a fluorescent screen 84. In the specific tube illustrated, the cathode ray is electrostatically focused.

From the foregoing description, it will be seen that, by employing my invention, all disturbances produced at the transmitter by reason of the return deflection of the cathode ray are prevented from having any effect upon the fluorescent screen 84 of the receiver.

I claim as my invention:

In a television system of the type employing a cathode ray transmitter tube at the transmit-I ter for producing picture signals and a cathode ray receiver tube at the receiver for reproducing the transmitted picture on a receiving screen, said tube at the transmitter being of the type including a screen comprising a mosaic of elements each capable of storing an electric charge, the method which comprises causing the cathode ray in said receiver tube to scan the receiving screen whereby it has a useful scanning period and a -return line period, causing the cathode ray of said transmitter tube to scan said mosaic over an area substantially greater than the area of the mosaic coveredy by the picture to be reproduced and in synchronism with the scanning of said receiving screen and with its useful scanning period greater than and with its return line period less than the corresponding periods at said receiver, and blocking the cathode ray in said receiver tube for a period which includes the beginning and the end of the useful deiiecting period of the cathode ray of said transmitter tube as well as the return line period of the cathode ray of said transmitter tube.

ALDA V. BEDFORD. 

